设A/3=B/4=C/5=K 所以A=3K B=4k c=5k 2(5k)-4k=126k=12 k=2再把K带入就OK了!!!
设A =3k b=4k c=5k2c-b=6K =12 k=2C=(3+4+5)*2 =24S=3*4/2=6